Temperature stable ceramic capacitors meeting the EIA standard, COG, (a.k.a. NPO), must exhibit a change in capacitance of within +/-30 ppm over the temperature range of from -55.degree. C. to 125.degree. C. Such capacitors also must have a quality factor, Q, that is greater than 1,000 measured at 1 megahertz, which is equivalent to the dissipation factor (DF) being no greater than 0.01. Ceramic precursor powders having been fired under conditions to achieve a mature ceramic at 95 percent or more of the theoretically possible density are most likely to provide such high quality dielectric ceramics. The sintering of high-firing ceramic precursor materials such as magnesium zinc titanate is typically achieved at a temperature of 1100.degree. C. It is well known to add a small quantity of glass containing flux as a sintering aid to the high-firing ceramic powders to reduce the sintering temperature necessary for yielding a mature dielectric ceramic. Examples of COG ceramic compositions sinterable at about 1100.degree. C. are found in patents U.S. Pat. No. 4,882,650 issued Nov. 11, 1989 and U.S. Pat. No. 4,533,974 issued Aug. 6, 1985.
When making a multi-layer capacitor (MLC), a metal sheet electrodes are interspersed between successive layers of the green precursor ceramic powder which includes the flux if any. Thus the buried metal electrodes in the green (unfired) ceramic must be subjected to a temperature high enough to sinter the ceramic to maturity. The most commonly used formulation for the metal electrode is by weight 70% Ag and 30% Pd. This formulation has a melting temperature of 1150.degree. C. and is typically used for the electrodes in MLC capacitors that are to be heated to temperatures no higher than 1140.degree. C. to avoid the risk that the metal would melt and run out. When using sintering furnaces that cannot be relied upon to hold the temperature to within that 10.degree. C. difference, an even lower furnace temperature setting must be used, and a further 10.degree. C. safety factor is typically used in MLC manufacturing.
The addition of larger amounts of the sintering flux will enable a lower MLC sintering temperature hut at the cost of a reduced dielectric constant (K) and other degraded performance measures such as Q. Known start formulations of ceramic precursor plus flux, that can he sintered to a high density at lower temperatures than about 1100.degree. C. are relatively rare, and the industrial use of such a start powder mixture is even more rare because the formulations and the sintering conditions become more critical leading to lower yields.
The cost of palladium is an order of magnitude greater than the cost of silver and palladium is typically the greatest cost factor in the manufacture of a MLC capacitor. One alternative to the use of palladium in a buried electrode is the use of a base metal such as nickel and or copper. However with base metal electrodes, sintering must be accomplished at less than the melting points of the base metal, which for nickel is 1453.degree. C. but for copper is 1083.degree. C. And sintering must be effected in a rare-oxygen atmosphere which greatly complicates the process. Control of a low oxygen pressure atmosphere itself leads to increased costs, and the choices of ceramic composition that do not become semiconducting due to loss of oxygen at sintering are severely limiting with respect to any particular dielectric ceramic performance that may be obtained.
The flux used in many air-fired MLC capacitors contains oxides of bismuth cadmium and lead which are especially effective for lowering the melting point of the flux. For a given amount of flux, this advantageously leads to a further reduction of the sintering temperature. However, these oxides tend to reduce the Q of MLC capacitors and bismuth can react with silver palladium electrodes leading to an even poorer quality factor. These volatile heavy metal oxides also contaminate the sintering ovens leading to irregular sintering results. Perhaps most importantly they represent a hazard to the environment and especially a health hazard to personnel involved in making both the ceramic powder and the MLC capacitors.
It is therefore an object of this invention to provide a ceramic powder for use in making multilayer ceramic capacitors meeting the COG standard, which start powder is capable of being sintered to maturity in an air atmosphere at a temperature of 1000.degree.+/-50.degree. C. so that the MLC may contain electrodes of a more silver rich and less costly composition e.g. 85% Ag/15% Pd.
It is a further object of this invention to provide such a ceramic powder that includes a high firing part comprised of a magnesium zinc titanate and a low firing flux part having essentially none of the hazardous heavy metal oxides of lead, bismuth and cadmium.